This invention relates to semiconductor strain gage bridges and more particularly to an improved apparatus and method for calibrating such a bridge.
The Wheatstone bridge is a very popular circuit configuration and has been extensively employed as the measurement circuit in conjunction with a wide variety of transducers. The bridge configuration has been employed with suitable transducers for the measurement of pressure, temperature and in a host of other applications.
In the field of pressure transducers the four-arm Wheatstone bridge is widely employed. See for example U.S. Pat. No. 4,236,137 issued on Nov. 25, 1980 to A. D. Kurtz, et al. and entitled SEMICONDUCTOR TRANSDUCERS EMPLOYING FLEXURE FRAMES, as well as many other patents showing a typical bridge structure.
In employing such a bridge configuration both the user and manufacturer would desire to calibrate the bridge as well as associated circuitry coupled to the bridge in order to determine the full operating range. The technique is referred to as shunt calibration. In this technique a shunt resistor is applied across the arms of the bridge or across an element and is operative to load the output of the bridge so that the bridge will provide a voltage which is conventionally 80% of full scale voltage with a given excitation voltage applied to the bridge. Of course other values of shunt calibration signal levels may be employed as is convenient for a given application. The shunt resistor is added externally and the process is to calibrate the bridge and the associated system. Thus the output of the system is calibrated from a zero point to a value of 80% of full scale. In regard to pressure transducer using a Wheatstone bridge array, this may correspond to a typical voltage level of zero to 100 millivolts.
Shunt calibration is a method of checking the response of a transducer and works well with conventional bridge configurations which use sensor devices that are not affected by or have minimal temperature effects such as conventional metallic or wire strain gage bridges. However the shunt calibration technique does not work well with piezoresistive sensors which are semiconductor devices. As is well-known, such devices exhibit a change in resistance with temperature. Therefore, in using semiconductor devices, the shunt resistor has a different effect at each different temperature. Accordingly, there is no technique presently understood which would conveniently enable one to shunt calibrate a semiconductor bridge configuration independent of temperature.
There are of course many prior art techniques which attempt to temperature compensate such a bridge, but they do not allow for shunt calibration independent of temperature. See for example, U.S. Pat. No. 3,245,252 entitled TEMPERATURE COMPENSATED SEMICONDUCTOR STRAIN GAGE UNIT issued on Apr. 12, 1966 to D. F. First, et al.
It is therefore an object of the present invention to provide an improved apparatus and method of shunt calibration of a semiconductor transducer independent of temperature.